Hitherto, porous membranes of synthetic resins have been used in many technical fields as separation membranes for gas separation, gas-liquid separation, solid-liquid separation, etc., or as insulating materials, lagging materials, sound insulators and thermal insulators. Among these, for separation membranes, the following properties are required as affecting the separation performances. First, the porous membrane is required to have an appropriate porosity in view of the separation efficiency and a uniform pore diameter distribution for a better separation accuracy. In addition, it is required to have a pore diameter optimum for an objective material to be separated. Further, the materials forming the membrane are required to have a chemical resistance to the objective material subjected to separation, weatherability, heat resistance, strength, etc. Further, the materials are required to have sufficient elongation at break and strength at break as mechanical strengths for use as the porous membrane.
From the above view point, conventionally developed porous membranes of polyolefin resins (e.g., JP-B 46-40119 and JP-B 50-2176) have left problems in respects of reverse washing and chemical resistance for ozone treatment after the use thereof as a separation membrane.
Vinylidene fluoride resins are excellent in weatherability, chemical resistance, heat resistance, strength, etc., and have been studied for their use as a porous membrane for separation. However, while the vinylidene fluoride resins have the above-mentioned excellent properties, they do not necessarily have desirable formability because of their non-adhesiveness and poor compatibility. In addition, development of porous membranes have been focused on the provision of a high porosity and a narrow pore diameter distribution for improving the separation performance, and no product having satisfactory mechanical strengths has been obtained. Accordingly, when a porous membrane is used as a filter membrane, a supporting membrane is superposed on the porous membrane to enhance the mechanical properties at present. Further, in the case of using a porous membrane as a separator of batteries, it is desired for the porous membrane to have sufficient mechanical properties, such as elongation at break and strength at break sufficient to be durable in a winding step in production of the batteries as the membrane is used in the form of being wound about a core material. Further, when used as a separator of batteries, the porous membrane is desired to have a narrow distribution range of penetrating pore diameters capable of preventing the passing therethrough of fine powdery active substances in the electrodes and a high efficiency in impregnation with an electrolytic solution which is performed after winding the porous membrane about the core material Further, when used as a microfiltration membrane, it is desired for the membrane to retain a high filtering performance for a long period.
As a process for producing a porous membrane of a vinylidene fluoride resin, JP-A 3-215535 has disclosed a process of mixing an organic liquid, such as diethyl phthalate, and hydrophobic silica as inorganic fine powder with a vinylidene fluoride resin, melt-forming the mixture and then extracting the organic liquid and inorganic fine powder. The thus-obtained porous membrane has a relatively large mechanical strength. However, as an alkaline aqueous solution is used for extracting the hydrophobic silica in the process, the vinylidene fluoride resin constituting the membrane is liable to be deteriorated.
On the other hand, our research group has also made several proposals of process for producing porous membranes of vinylidene fluoride resin used as a microfiltration membrane or a separator of batteries. Those are, for example, a process of subjecting a vinylidene fluoride resin to steps of crystallization, heat treatment, stretching and heat treatment under tension, thereby forming a porous membrane (JP-A 54-62273); a process of forming a film of a vinylidene fluoride resin of a specific molecular weight together with a plasticizer, cooling the film from one side thereof and then extracting the plasticizer (JP-A 7-13323); a process of blending with a vinylidene fluoride resin of an ordinary molecular weight, a vinylidene fluoride resin of a high molecular weight for providing an increased heat distortion resistance and an organic pore-forming agent or an inorganic pore-forming agent, forming a film of the blend and then converting the film into a porous membrane by removing the pore-forming agent by extraction or by stretching the film with the inorganic pore-forming agent as stress-concentration nuclei during the stretching in the case of using such an inorganic pore-forming agent (JP-A 2000-309672); etc. However, in the case of extraction of a plasticizer or an organic pore-forming agent, the resultant porous membrane is liable to fail in exhibiting filtering performance (water permeation rate or permeability) or mechanical properties required when the porous membrane is used as a filtering membrane. On the other hand, when the stretching of the membrane is tried in order to improve these properties, the membrane is liable to be severed so that a sufficient ratio of stretching cannot be effected. Particularly, in the case of being used as a microfiltration membrane, the membrane generally has a thickness of at least 50 μm so as to be durable against the filtration pressure, whereas the stretchability of such a relatively thick membrane having a thickness of at least 50 μm becomes inferior remarkably.
Consequently, there has not been actually obtained a porous membrane of vinylidene fluoride resin which has fine pores of appropriate size and distribution, also has excellent mechanical strengths and is therefore suitable as a microfiltration membrane or a separator of batteries.